The present invention relates to a highly efficient water distillation process and an apparatus thereof and more particularly, the present invention is directed to a highly efficient water distillation process which minimizes fouling and scaling of operating equipment over long periods of operation.
Generally speaking, water distillation is a highly effective method of vaporizing a pure water distillate and recovering a concentrated liquid containing a large quantity of non-volatile components. This process method can be an effective means to recover clean pure water from contaminated sources. However, water distillation processes typically have several problems not the least of which can be fouling or scaling of the apparatus with minerals or other components from the fluid being distilled. Common scaling compounds consist of calcium, magnesium and silicon. Fouling, or to a greater extent, scaling of the heat transfer surfaces have a detrimental effect on the capacity of the heat transfer components, causing conventional distillation processes to become inoperable.
Another common problem with typical water distillation processes is that the high energy input requirements. Without a means to effectively recover the input energy, the energy required is equivalent to the latent heat of vaporization of water at a given pressure/temperature. Water distillation, under this condition is not commercially viable for water remediation applications.
One of the most difficult factors in processes of this kind is encountered when increased concentrations of insoluble solids are realized.
Several variables must be considered to overcome the problems with conventional distillation methods. The following three equations describe the basic heat transfer relationships within a water distillation system:
where
In order to have an efficient distillation system, the quantity of heat exchanged and recovered, Q, expressed by the above stated equations, must be maximized, while at the same time obeying the practical limits for the remaining variables and preventing scaling and fouling. For a given fluid and fluid dynamics within a given heat exchange apparatus, the variables, U, Cp and L are relatively non-variable. Therefore, careful consideration must be given to the variables A, Q/A, LMTD, m, and T1 and T2 to overcome the problems associated with distillation of contaminated water.
To fully overcome the problems related to distilling contaminated water and eliminate scaling, other essential factors must be considered beyond the basic equations stated above:
the rate by which the heat is transferred within the distillation system, known as heat flux or QAxe2x88x921 (Btu hrxe2x88x921 ftxe2x88x922)
the level of contaminates in the concentrate;
the final boiling point of the concentrate relative to the saturation temperature of the vapor stream;
the degree of supersaturation and level of precipitation of the concentrate; and
level of vaporization of the evaporating stream.
One of the most difficult problems in distillation as set forth herein is realized with increased concentrations of insoluble solids. Generally, dissolved solids in the feed stream concentrate in the separator which gives rise to a boiling point rise (BPR) supra. This results in a requirement for greater energy input thus leading to compromised system efficiency.
Until the advent of the present invention, maximizing the quantity of heat transferred and recovered with a water distillation process, without the tendency of fouling or scaling, could not be realized over a long term continuous period.
A process which is both energy efficient and eliminates the problems of scaling previously encountered in the distillation of contaminated water, contaminated with organics, inorganics, metals, inter alia., and which has applicability in the remediation industry.
The invention is predicated upon the marriage of two distinct concepts, both of which have been previously identified singularly in the prior art but which have not been uniquely configured with the synergistic effect that results with the present invention. It has been found by employing a conventional vapor recompression circuit together with a uniquely configured forced convection heat recovery and transfer circuit, that very desirable results can be obtained in terms of maximizing heat transfer and maintaining the desired forced convection circuit non-conductive to scaling exchangers, which is typically encountered by practicing standard distillation methods.
One object of the present invention is to provide an improved efficient process for distilling water containing organic, inorganic, metals or other contaminant compounds with the result being a purified water fraction devoid of the contaminants which additionally does not involve any scaling of the distillation apparatus.
According to one aspect of the present invention, there is provided a method of removing contaminants from a fluid feed stream containing solid and liquid contaminants by mechanical vapor recompression employing a heated separator, heat exchanger and compressor, the method comprising the steps of:
a) providing a feed stream;
b) providing a nonaqueous liquid with a heat capacity and boiling point greater than water;
c) heating the feed stream in the heated separator to generate a vapor fraction substantially devoid of contaminants and a liquid contaminant fraction containing solid nonevaporated contaminants;
d) precipitating the solids in the nonaqueous liquid to prevent accretion in the separator and prior to contact with the heat exchanger; and
e) contacting the vapor fraction with the exchanger to provide a condensed distillate devoid of contaminants, whereby the solids do not energetically affect condensation of the distillate.
It has been found that by precisely controlling the ratio of circulating mass in a range of less than 300 to near two times that of the vapor fraction being compressed, several desirable advantages can be realized:
1. The circulating concentrate through the evaporating side of the reboiler will contain a precisely controlled vapor fraction near 1% to 50% of the mass of the circulating concentrate;
2. By precisely controlling this vapor fraction, the temperature rise of the circulating concentrate remains very low (about 1 F.) and cold heat exchange surfaces remain wetted, at a temperature near that of the circulating fluid. This reduces the risk of fouling of these surfaces;
3. With this controlled low vapor fraction, the concentrated fluid within the exchanger is subjected to an additional localized concentration factor of less than 1.1, avoiding localized precipitation of scaling compounds;
4. As the vapor fraction increases and the concentration factor increases while passing through the reboiler, the stream velocities increase significantly thus reducing the risk of fouling;
5. By allowing a controlled vapor fraction in the evaporating fluid, significant heat transfer can be realized through the means of latent heat, without scaling;
6. Because the temperature rise of the evaporating side of the reboiler is kept very low, the LMTD of the reboiler is maintained, thereby keeping the compression energy very low; and
7. By adjusting the heat flux, the temperature of the wet surfaces for condensing and evaporating are maintained near that of the saturated steam condition. The type of boiling experienced will range from primarily forced convection to stable nucleate boiling of the wetted surfaces.
In accordance with a further aspect of the present invention, there is provided a method of removing liquid and solid contaminants from a fluid feed stream containing the contaminants, comprising the steps of:
a) providing a feed stream containing water;
b) providing a nonaqueous liquid with a heat capacity and boiling point greater than the water;
c) heating the feed stream in the heated separator to generate a vapor fraction substantially devoid of contaminants and a concentrate liquid contaminant fraction containing solid nonevaporated contaminants;
d) precipitating the solid nonevaporated contaminants in the nonaqueous liquid prior to contact with the heat exchanger;
e) contacting the vapor fraction with the exchanger to provide a condensed distillate;
f) compressing the vapor fraction to generate a temperature differential in a reboiler exchanger;
g) passing the vapor fraction into contact with the reboiler exchanger to provide a condensed distillate from the reboiler;
h) circulating at least a portion of the concentrate through the reboiler exchanger and the heated separator to maintain a ratio of circulating mass to vapor mass of about 300 to about near 2; and
Broadly, in one possible embodiment, distilled water is evaporated and passed through a mesh pad to remove any entrained water before entering the compressor. The compressor elevates the pressure and temperature of the vapor stream above that of the heated separator to allow effective heat transfer across the reboiler heat exchanger. The vapor stream subsequently enters the reboiler where it is xe2x80x9cdesuperheatedxe2x80x9d and condensed to distillate. The heat energy is transferred to the circulating concentrate from the heated separator where, by way of controlling the mass of circulating concentrate to vapor stream, to a range of less than 300 to near 2, less than 50% vapor, more precisely less than 10%, vapor is generated in the circulating concentrate stream. This vapor phase absorbs the transferred heat by latent heat of vaporization, while at the same time not allowing the temperature rise on the circulating concentrate to increase greater than about 1 F. The clean distillate water at condensing temperature and pressure, passes through the preheater to recover the sensible heat portion of the system to the incoming feed stream. Simultaneously, a portion of the concentrate stream is removed from the heated separator to control the desired concentration of contaminants. This concentrate blowdown stream at the heated separator temperature and pressure, is passed through an additional preheater to impart the remaining sensible heat energy to the feed stream. Additional pre and post-treatment techniques can be employed as batch or continuous process methods to remove or contain contaminants during the distillation operation, pH control methods can be used to ionize volatile components or alter solubility conditions in the concentrate to further enhance the subject distillation process.
The distillate water recovered can be controlled to purity level and temperature level which allows it to be reused as process water, reused as distilled water or released to nature water sheds meeting or exceeding virtually all environmental water quality standards.
In terms of the breadth for this process, the same could be easily employed to decontaminate industrial processed water such as that in the refinery, petrochemical, pulp and paper, food, mining, automotive/other transportation industries and the manufacturing industries. In addition, applications are envisioned for landfill leachate water, desalination, ground water remediation, well water cleanup, lagoon remediation, oil field waste water recovery, as well as producing any form of boiler feed water, and concentrating valuable components from dilute streams. This listing is by no means exhaustive, but rather exemplary.
Having thus described the invention, reference will now be made to the accompanying drawings illustrating preferred embodiments.